In recent years, in the fields of a biosensor etc., various sensors using surface plasmon resonance have been put in practical use. The examples of a sensor using Surface Plasmon-field enhanced Fluorescence Spectroscopy (SPFS) include one, wherein a metal film is formed via a thin substrate on an inclined plane of a prism, and the metal thin film is irradiated with a laser beam from a back side, which is the inner side of the prism, to generate surface plasmon resonance and an enhanced electric field at the surface of the metal thin film (see Patent Literature 1). Note that this Patent Literature 1 describes that the average roughness Ra of the surface of the metal film is set to be equal to or less than 4 nm.
Moreover, the examples of a scanning near-field optical microscope utilizing surface plasmon resonance include the one, wherein a metal thin film is formed via a thin substrate on an inclined plane of a prism, and the metal thin film is irradiated with a laser beam from a back side, which is the inner side of the prism, to generate surface plasmon resonance at the surface of the metal thin film, thereby detecting a proximity light with an opposed probe (see Patent Literature 2). Note that this Patent Literature 2 describes that the average roughness of the surface of the metal thin film is desirably set to be equal to or less than 0.1 nm.
Other than the above, the examples of a surface plasmon resonance apparatus, which causes a light beam to enter the opposite side of a sample of a metal film and detects an attenuation state of a totally reflected light thereof, include the one having an incident face and emitting face provided in the bottom portion of a container-like chip to be measured and having a metal film formed in the upper face of the bottom portion (see Patent Literature 3). Note that this Patent Literature 3 does not describe the surface roughness of a metal film.
In an apparatuses utilizing the surface plasmon resonance phenomena as described above, the metal thin film of a sensor might be desirably smooth, but according to the study of the present inventor, it turned out that a reduction of the average roughness of the metal thin film does not necessarily result in an improvement of the S/N ratio.